1. Field of the Invention
The present invention pertains to reduction-stable COG ceramic compounds having a high dielectric constant.
2. Discussion of the Related Art
Such COG ceramic compounds are utilized, for example, in multi-layer capacitors and LC filters with copper electrodes and are suitable for applications in the high-frequency range. What are referred to as COG ceramic are compounds that have a low ( less than 30 ppm/K) temperature coefficient of the capacitance.
In the case of materials with a low dielectric constant DK, technical solutions have already been disclosed that allow the common sintering of a ceramic compound with Cu electrodes under reducing conditions in that the sintering temperature is lowered below the melting temperature of the copper (1083xc2x0 C.). Specific sintering aids, preferably additives of glass frit, whose material basis is systems containing lead oxide and/or bismuth oxide are applied. A partial oxygen pressure less than 10xe2x88x922 Pa must be applied in order to suppress the oxidation of the copper during the sintering in the region of 1000xc2x0 C.
At the same time, a lower, critical limit value of the partial oxygen pressure dare not be downwardly transgressed since, otherwise, the ceramic or a constituent of the added glass frit is subjected to reduction, which necessarily leads to a lowering of the insulation resistance and to an unacceptable increase in the dielectric losses. The de-bindering of the green compact must be completely
The invention is directed to reduction-stable COG ceramic compounds having a high dielectric constant according to the preamble of patent claim 1.
Such COG ceramic compounds are utilized, for example, in multi-layer capacitors and LC filters with copper electrodes and are suitable for applications in the high-frequency range. What are referred to as COG ceramic are compounds that have a low ( less than 30 ppm/K) temperature coefficient of the capacitance.
In the case of materials with a low dielectric constant DK, technical solutions have already been disclosed that allow the common sintering of a ceramic compound with Cu electrodes under reducing conditions in that the sintering temperature is lowered below the melting temperature of the copper (1083xc2x0 C.). Specific sintering aids, preferably additives of glass frit, whose material basis is systems containing lead oxide and/or bismuth oxide are applied. A partial oxygen pressure less than 10xe2x88x922 Pa must be applied in order to suppress the oxidation of the copper during the sintering in the region of 1000xc2x0 C. At the same time, a lower, critical limit value of the partial oxygen pressure dare not be downwardly transgressed since, otherwise, the ceramic or a constituent of the added glass frit is subjected to reduction, which necessarily leads to a lowering of the insulation resistance and to an unacceptable increase in the dielectric losses. The de-bindering of the green compact must be completely realized before the onset of the sintering in order to avoid a local downward transgression of this lower critical limit value.
The publications EP 0 534 802 A1, U.S. Pat. Nos. 5,264,403, 5,304,521, 5,350,721, 5,479,140, 5,493,262, 5,488,019 and 5,485,132 disclose ceramic compounds of the substance systems BaOxe2x80x94TiO2xe2x80x94SE2O3 wherein the oxide of the rare earth metals SE2O3 can be partially replaced by Bi2O3 whose sinter compression already partly succeeds at 900xc2x0 C. in that glass frit parts that contain CdO, PbO or Bi2O3 or glasses of the system ZnOxe2x80x94B2O3xe2x80x94SiO2 are added. A common sintering with silver electrodes in air is thereby enabled. For a common sintering with copper electrodes under inert conditions, for example under nitrogen, the systems prove to be inadequately stable with respect to a partial reduction, which results in a lowering of the insulation resistance and increase in the losses.
The publications EP 0 534 801 A1, U.S. Pat. Nos. 5,458,981 and 5,292,694 have likewise protected BaOxe2x80x94TiO2xe2x80x94Se2O3 ceramic compounds in combination with glass additives containing B2O3 and ZnO for the purpose of the common sintering with silver electrodes. In these cases, too, the de-bindering under air access prevents the combination with copper electrodes, so that there must be recourse to silver or silver/palladium alloys as electrode material. The advantage of a cost-beneficial application of silver electrodes is opposed by the disadvantage of great mobility, particularly at high temperature, this leading to migration effects and causing a deterioration of the dielectric properties as a result.
In the earlier German Patent Application 19749858.2, the material system BaOxe2x80x94PbOxe2x80x94Nd2O3xe2x80x94TiO2 used for the manufacture of high-DK COG capacitors and microwave resonators in the region of the phase formation of rhombic bronzes (Ba1xe2x88x92yPby[sic])6xe2x88x92xNd8+2x/3Ti18O54 with 0.6 less than x2.1 and 0 less than y less than 0.6 is made available for a sintering at temperatures less than 1030xc2x0 C. and, thus, for the common sintering with Cu electrodes in that the sintering aid, preferably PbO-free glass frit having a specific composition is added and a complete de-bindering under nitrogen is achieved by the action of water steam at elevated temperature upon utilization of the steam reforming process known from petroleum processing. The matching of the temperature coefficient of the capacitance TKC or, respectively, of the temperature coefficient of the resonant frequency TKxcexd0 to the tolerance limits valid for COG capacitors TKC less than 30 ppm/K or, respectively, LC filters with TKxcexd0 less than 10 ppm/K succeeds in that modifications of the composition of the ceramic compound are undertaken with additives of Nd2Ti2O7 or of Ba4.5Sm9Ti18O54. It should be noted as a limitation to this technical solution that the stability of the ceramic is limited due to the PbO content, which requires an especially careful de-bindering and the avoidance of too low a partial oxygen pressure in the sintering. The two demands are linked to one another since, in particular, the unacceptable downward transgression of the critical partial oxygen pressure limit, caused, for instance, by slight organic residue constituents, must also be avoided locally. Otherwise, a eutectic Pb/Cu alloy forms which melts at 924xc2x0 C., i.e. the electrodes dribble out.
Systems free of PbO and Bi2O3 prove suitable for avoiding such a disadvantage. Thus, for example, the ceramic Ba4(Sm0.6Nd0.4)9.233Ti18O54 that can be presented by sintering at approximately 1400xc2x0 C. is characterized by a TKxcexd0=value of +9 ppm/K and a quality-frequency product Qxcexd0=9000 (3 GHz) [P. Lafez, G. Desgardin, B. Raveau, J. Mater. Sc. 30 (1995) 267]. Comparable values are recited in the literature for the composition Ba4.2(Sm0.3Nd0.7)9.2Ti18O54 [H. Ohsato, H. Kato, M. Mizuta, S. Nishigaki, T. Okuda, Jpn. J. Appl. Phys. 34 (1995) 5413]. For lowering the high sintering temperature required for ceramic compounds of the system BaOxe2x80x94TiO2xe2x80x94Nd2O3xe2x80x94Sm2O3, EP 0 701 081 A1 proposes glass frit additives that contain PbO and even GeO2, which is disadvantageous for the stability with respect to reduction for the reasons that have been presented.
The invention is based on the object of manufacturing COG ceramic compounds with optimally high dielectric constant on the basis of a substance system that is highly stable with respect to reduction and that allows a TKC value less than 30 ppm/K or a TKxcexd0 value less than ppm/K to be set dependent on the selected composition, and to also specify a sintering temperature that lies under the melting temperature of the copper and enables the sintered compression in the presence of inside copper electrodes upon preservation of the dielectric properties needed for COG multi-layer capacitors and LC filters.
This object is inventively achieved with a reduction-stable COG ceramic compound that comprises the features of patent claim 1.
Advantageous developments are recited in the subclaims. The advantage of the invention is comprised in utilizing the substance system BaOxe2x80x94Nd2O3xe2x80x94Sm2O3xe2x80x94TiO2 used for the manufacture of COG capacitors and microwave resonators with a high DK in the region of the phase formation of rhombic bronzes Ba6xe2x88x92x(SmyNd1xe2x88x92y)8xe2x88x922x/3Ti18O54 in that a TKC value less than 30 ppm/K is achieved in the series Ba4.5(SmyNd1xe2x88x92y)9Ti18O54 (x=1.5) for y=0.9 or, respectively, in the series Ba4(Sm0.7Nd0.3)9.33Ti18O54 (x=2) for y=0.7 and in that a TKxcexd0 value less than 10 ppm/K can be designationally set in the series Ba4.5(SmyNd1xe2x88x92y)9Ti18O54 (x=1.5) for y=0.9 or, respectively, in the series Ba4(Sm0.7Nd0.3)9.33Ti18O54 (x=2) for y=0.8, and to simultaneously enable a sintering at temperatures less than 1030xc2x0 C. and, thus, for the common sintering with Cu electrodes in that a sintering aid, preferably a glass frit having a specific composition, is added to such a ceramic powder.
It is also advantageous that, on the basis of the knowledge of the composition-dependency of the TKC or, respectively, TKxcexd0 values in said series, a shift of the temperature coefficient toward positive or negative values caused by the additive of glass frit can be compensated by a designational change of the composition.
The advantage of the manufacturing method is comprised therein that the sinter compression is implemented in nitrogen given a partial oxygen pressure less than 10xe2x88x922 without the properties that are typical for COG capacitors and microwave ceramics and, thus, for LC filters being lost, for instance as a result of a partial reduction.
The complete de-bindering of the green compact succeeds in a temperature range below the inception of the sinter compression in that process known from petrochemicals for decomposing hydrocarbons or more highly condensed organic compounds as well to form carbon dioxide and hydrogen due to the influence of water steam at elevated temperature (xe2x80x9csteam crackingxe2x80x9d) is transferred onto the ceramic process. For example, a slight, negative free enthalpy can be estimated from thermodynamic data for the decomposition of polyethylene glycol or polyacrylic acid as binder according to the reaction: 
so that the process of de-bindering the green compacts, which must be undertaken for the purpose of avoiding an oxidation of the copper in nitrogen (partial oxygen pressure less than 10xe2x88x922 Pa), can completely occur.
The invention is explained in greater detail on the basis of the following exemplary embodiments.